Highest bidder probably isn't the best bidder
An auction has no merit whatsoever.
We're not talking about a bit of ancient pottery here, we're talking about something that should be of value to society as a whole, not just the company that pays for the spectrum.
The company that bids the most is more likely to be the company that's going to charge the end-users the most, rather than the company that's going to make the best use for it.
Frequency should be allocated according to need and merit, not superficial profit to the government.